The present invention relates to a piezoelectric polymer material having a high piezoelectric performance even at a frequency in a high frequency range, and more particularly, a polarized, shaped bodies comprising a copolymer of vinylidene fluoride for use as elements in an ultrasonic transducer, the ultrasonic transducer provided with the elements being superior to the conventional ultrasonic transducer provided with conventional elements in sensitivity of transmitting and receiving ultrasonic waves.
As a piezoelectric polymer substance, for instance, poly(vinylidene fluoride) and a copolymer of vinylidene fluoride have been reported, respectively in Japanese Patent Publications Nos. 45-83771 (1970) and 50-29159 (1975), and it has been reported in Japanese Patent Publication No. 51-23439 (1976) that these polymer and copolymer have a piezoelectric performance even at a frequency in a high frequency region.
Furthermore, in Japanese Patent Application Laying Open No. 53-26995 (1978), it has been disclosed that a binary copolymer of vinylidene fluoride and trifluoroethylene is a piezoelectric polymer material.
Among these fluoropolymers, poly(vinylidene fluoride) has been considered to be the most useful piezoelectric polymer material for use as elements in an ultrasonic transducer because in the case where a stress or an electric field is loaded onto the shaped body in the direction parallel to the direction of polarization, the electromechanical coupling factor (k.sub.t) being 0.20 and of the processability to shaped bodies being favorable. However, a further improvement of its performance has been demanded with the requirement for higher sensitivity of the transducer.
A higher electromechanical coupling factor (k.sub.t) can be obtained in a binary copolymer of vinylidene fluoride, particularly in a copolymer of 75 mol % of vinylidene fluoride and 25 mol % of trifluoroethylene. However, in the case where the binary copolymer is subjected to shaping into elements having a concave for improving the ultrasonic sensitivity of the transducer, cracks are frequently formed in the shaped elements to remarkably reduce the production yield of the elements. In addition, a large improvement of the ultrasonic sensitivity of the transducer provided with the thus prepared element corresponding to the large value of k.sub.t thereof was not obtained.
The object of the present invention is to provide a piezoelectric polymer material giving a high yield in preparing shaped elements therefrom for use in an ultrasonic transducer and giving a high ultrasonic sensitivity to the ultrasonic transducer provided with the thus prepared element.
As a result of studying and examining various copolymers derived from vinylidene fluoride, the present inventors have found that a copolymer of vinylidene fluoride, trifluoroethylene and vinyl fluoride shows a high yield in preparing elements for the ultrasonic transducer therefrom and a high electromechanical coupling factor (k.sub.t) and gives a high sensitivity to the ultrasonic transducer provided with the thus prepared element, and have attained the present invention.